The present invention relates to a method for interference cancellation in a cellular CDMA network served by base stations, in which method the interference caused by mobile stations that are in the neighboring cells is accounted for in the base station receivers.
In a cellular CDMA network, in which base stations serving adjacent cells use the same carrier frequency to communicate through mobile stations, interference occurs during handover as mobile stations move from one cell to another until the latter cell has taken over communication with the mobile station. This interference is caused by power adjustment in the communication between the base stations and the mobile stations; power adjustment aims at good reception in each cell regardless of new mobile stations entering the area from neighboring cells. In such a case, for example, before the handover a mobile station approaching a neighboring base station is, as far as the base station is concerned, a source of interference with increasing signal strength, forcing the base station to increase its own transmitting power and/or that of the mobile stations within the cell so that it becomes higher and higher resulting in an increase in the total noise level of the network. In any case, without interference cancellation, the situation leads to repeated overrated transmission power levels, which is most disadvantageous to communication quality and channel use optimization in a CDMA network.
These problems occur especially in using a so-called hard handover technology in which only one base station at a time is communicating with a mobile station. Hard handover represents conventional technology that is relatively easy to implement. The problem in a CDMA network is the interference situations caused by multiple use of one carrier frequency. The interference situations can be controlled by different kinds of interference cancellation routines which measure the interfering signal and generate a cancelling signal which, when combined with the receiver input signal, eliminates the interfering signal from it. This procedure is repeated when necessary to eliminate several interfering signals, c.f. for example WO Application 92/11722 and European Patent Application 491668. The methods developed for the cancellation of interfering signals are largely at a theoretical stage and very complicated. Furthermore, they require that all the base station receivers separately measure all interfering signals in order to determine how to cancel interference from the input signal.
In efforts to reduce interference in handover situations, a so-called soft handover technology has also been employed. This technology allows two base stations to communicate simultaneously with a mobile station currently switching cells and thus eliminates sources of interference even before a mobile station geographically enters the new cell area. As far as the network is concerned, the implementation of the soft handover technology is rather an expensive and complicated operation which requires, among other things, spare channels for performing the handover, and a freely allocatable diversity structure in the mobile station receiver to enable dual connection.